DESCRIPTION (Applicant's Abstract): Cannabinoids, the principal psychoactive constituents of marijuana, have profound effects on mood, memory, movement, and nocioception. Acting via the neuronal cannabinoid receptor (CB1), these compounds are potent modulators of ion channel function. Specifically, they inhibit N- and Q- type voltage-dependent calcium channels and activate inwardly-rectifying potassium channels. These actions suggest that cannabinoids may elicit their behavioral effects by decreasing neuronal excitability and neurotransmitter release from CB1- expressing neurons. During the next five years we propose to study in detail the molecular events involved in cannabinoids inhibition of neurotransmitter release and neuronal excitability, the modulation of these responses by protein kinase C, and the early molecular events associated with the development of tolerance to cannabinoids. These studies involve three major aims: 1. Defining the extent and mechanisms of inhibition of neurotransmitter release by cannabinoids in neuronal cultures using electrophysiological and imaging techniques. 2. Delineating the role of protein kinase C (PKC) in cannabinoid signaling. Recently we have found that ion channel modulation by cannabinoids is strongly disrupted by protein kinase C activation and that CB1 receptor is a substrate for PKC. We will determine if CB1 phosphorylation occurs in situ, if nurotransmitter activation of PKC disrupts CB1 signaling, and if PKC activation prevents cannabinoid-mediated inhibition of neurotransmitter release. 3. Identifying the molecular events associated with the development of cannabinoid tolerance. These studies will significantly advance our understanding of the cellular actions of cannabinoids. There are three reasons why this is imnportant: (1) Interest in marijuana is increasing and is a significant social issue (witness the ballot initatives in a number of states); (2) cannabinoids have therapeutic potential and we need a firm appreciation of their cellular physiology to exploit this potential; (3) we know little about the physiological role of endogenous cannabinoids in health or diseased brain.